1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to devices and systems used for marine exploration and, more particularly, to deflectors that are attached to cables of marine seismic survey systems via a knuckle portion, with the deflectors providing lift forces to enhance stability of the survey geometry.
2. Discussion of the Background
During the recent past, interest in developing new oil and gas production fields has dramatically increased. With availability of land-based production fields being limited, offshore drilling locations that appear to hold vast amounts of fossil fuel have developed. Since offshore drilling is an expensive process those engaged in it invest substantially in geophysical surveys to determine promising drilling locations based on more accurate information in order to avoid dry wells.
In geophysical surveys, waves (such as, but not limited to, seismic waves) are directed toward the seabed. Information (e.g., travel time and energy) about waves reflected back to receivers located close to the water's surface is used to learn about the structure and nature of layers underneath the seabed (e.g., to generate images of the substructure). In order to obtain high-resolution images, multiple techniques and devices are used to maintain the data acquisition geometry (e.g., relative positions of the wave sources and the receivers) while performing the survey.
As illustrated in FIG. 1, in a marine seismic survey system 100, a vessel 110 usually tows a wave source 120, and streamers 130 (only one shown in FIG. 1) connected to the vessel 110 via lead-in cables 125. The streamers 130 are configured to carry wave detectors (i.e., receivers), such as 140a, 140b, 140c, 140d and 140e. 
Waves generated by the source 120 penetrate the seafloor 150 and layers 155, 165 and 175 underneath. The waves may propagate at different speeds through these layers 155, 165 and 175, because the layers have different properties (e.g., composition and density). Reflection and refraction may occur when waves cross interfaces between layers through which they pass at different speeds. Waves partially reflected at a first interface 160 between layer 155 and layer 165, at a second interface 170 between layer 165 and layer 175, etc., are detected by receivers 140a-140e located along the streamers 130.
Currently used marine survey systems include plural streamers. It is desirable to deploy and maintain the streamers and the source(s) at predetermined depths and relative lateral offsets, according to the intended data acquisition geometry. One such device used to arrange and maintain the components of the marine survey systems according to the intended data acquisition geometry is a deflector 135. In FIG. 1, the deflector 135 is attached to the lead-in cables 125.
Deflectors of different sizes may be deployed at different positions along a cable to provide a lift force while towed underwater. Plural deflectors may be attached to the same cable, or one deflector may be connected via cables or ropes so as to affect plural streamers.
As illustrated in FIG. 2, a conventional method of attaching a deflector 200 (which has a rectangular wing body 210) to a lead-in cable 225 located between the towing vessel (not shown) and the streamer 230, uses more or less complex rigging 227 connected usually to one side of the wing body 210. A disadvantage for this conventional method is that the deflector's capacity to recover and return to its intended posture (e.g., the desired attack angle) following an accidental or intentional departure thereof is low.
Another conventional method of attaching a deflector to a cable uses a pivot link. A disadvantage of this conventional method is the high risk of the deflector rotating around the pivot and towing the cable down rather than laterally when an occasional pitch movement (which is coupled with the roll) occurs.
Thus, it is desirable to have a deflector with a more flexible connection to the lead-in cable, which deflector overcomes problems associated with the conventional deflectors.